Battery systems containing lithium ion battery cells or other plural battery cells (secondary battery cells) connected in series are used in various pieces of equipment. Since the battery structure is small but has a high output, a charge/discharge abnormality may take place easily. There are various types of charge/discharge abnormalities, such as rising voltage due to over-charging of the cells, falling voltage due to over-discharge, over-charging current flowing from the external path to the battery cells, over-discharge current flowing from the battery cells to the outside, etc. When such an abnormality takes place, it is necessary to detect it appropriately and to protect the battery cells and the external equipment from over-currents and over-voltages.
Recently, the performance of battery systems has been improved by the inclusion of various functions, such as the remaining charge display function, etc. As a result, a battery system that has a microcomputer (μC) to control plural integrated circuits (IC) has been realized in many schemes. In such cases, it is also necessary to protect the IC that handles the small signals from over-charging or over-discharging.
In consideration of said requirement, the protection function is important. However, if the protection function is too strict, the current path will be cut off frequently due to noise, etc., making it difficult to use. On the other hand, it is necessary to apply several levels of fail-safe features to ensure that there is no fatal damage that may destroy the battery cells and external equipment. Therefore, protection is increased by the inclusion of different protection levels, that is, primary and secondary levels. Also, efforts have been made to use separate ICs for primary protection and for secondary protection, so that different abnormalities in the system, such as in the power source, clock signal, etc., do not influence each other.
However, separating the ICs used for different protection levels is undesirable in consideration of assembly size and cost. Also, when it is determined that the primary protection function is down, a fuse must be blown to initiate the secondary protection function. Such battery system must be reset to be used again, which is undesirable. That is, although separating the ICs in accordance with the protection level has the advantage of increased protection, there is the disadvantage that when it is determined that the primary protection function is down due to a malfunction, etc., the system cannot be reset. Also, this scheme is undesirable in consideration of size and cost.
A problem for the battery system is that when the protection function is increased, the protection function may work unnecessarily, but then cannot be reset. As a result, the operating efficiency is poor.